ldns-config: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/ldns-config - from package: ldns-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/ldns-config - from package: libldns-dev

You may install the required package with command apt-get install {package}

Error in other languages:
ldns-config: bevel nie gevind nie
ldns-config: comandă negăsită
ldns-config: comando não encontrado
ldns-config: commande introuvable
ldns-config: command not found
ldns-config: käsku ei ole
ldns-config: không tìm thấy lệnh
ldns-config: komanda nerasta
ldns-config: Kommando nicht gefunden.
ldns-config: kommandot finns inte
ldns-config: komut yok
ldns-config: nie znaleziono polecenia
ldns-config: níor aimsíodh an t-ordú
ldns-config: no se encontró la orden
ldns-config: no s'ha trobat l'ordre
ldns-config: opdracht niet gevonden
ldns-config: parancs nem található
ldns-config: perintah tidak ditemukan
ldns-config: príkaz nenájdený
ldns-config: příkaz nenalezen
ldns-config: команда не найдена
ldns-config: командата не е открита
ldns-config: 命令找不到
ldns-config: コマンドが見つかりません